Explained: How does GPS actually work?

GPS is one of those technologies that we really take for granted. Google Maps, sat-nav, running watches, we're quick to get annoyed when they're slow or ever-so-slightly inaccurate but we never take the time to appreciate our Global Positioning System for what it is - majestic and kind of magic.

Here's how GPS actually works and why it's damn important to wearable tech.

Satellites gone, up to the skies

We'll start in space because, why not? Currently orbiting the earth are (at least) 24 satellites, solar powered, and placed there by the U.S. Department of Defence in the 70s. The satellites are orbiting at a speed of 7,000 mph approximately 12,000 miles above the earth's surface and in the 1980s, they became available for civilian use - that's you and me.

So how do satellites in space work out whether you're on Brewer Street or Frith Street? Well, each satellite orbit the earth in a very precise path twice a day and send microwave signals to GPS receivers which take this data and use triangulation to determine your exact location. At any one time a receiver needs to detect at least three satellites for latitude and longitude and four satellites to add your altitude.

What is triangulation? Essentially, the receiver in your smartphone or running watch takes the time the signal was sent from the satellite and the time the receiver receives it and uses that difference to work out how far away the satellite is. It repeats this distance and time lag measurement for further satellites in relation to the receiver (you) and can then work out your exact position and track this as you move. Clever.

The GPS satellites aren't actually alone up there. A rival system named GLONASS uses 24 Russian Aerospace Defence Forces satellites - some smartphones and wearables include both receivers to increase the chances of your precise location being detected and speed the whole sending signals and measuring distances process up by as much as 20%.

We got you - within 15 metres or so

GPS works 24/7 and in all weather conditions though the actual low power 1575.42Mhz radio signal travels by line of sight. So that means it won't go through solid buildings but it will pass through clouds and glass.

GPS is actually very accurate though, even in dense cities though it doesn't work indoors - a first world problem, tech like iBeacons is trying to rectify. Outdoors, standard GPS receivers, for instance, are accurate to within 15 metres.

As well as harnessing GLONASS satellites and using parallel receivers for higher accuracy, wearables and watches can also use WAAS (Wide Area Augmentation System) which can get you location down to within 3m or DGPS - Differential GPS which corrects GPS signals down to a range of 3 - 5m using a series of beacons.

It's worth noting that GPS has its own control and monitoring stations - four around the world.

The actual information transmitted includes an ID code for the satellite, the current date and time from the satellite's atomic clock, the status of the satellite and orbital info that shows where the satellite (and the rest of them) will be at any given time.

And as you'll probably realise once you've got an accurate GPS signal, it can be used to accurately track running speed, distance to a destination, sunrise and sunset.

So why doesn't it always work perfectly?

We've all been there at the start of a run, waiting for GPS to lock on, looking for satellites. Or realising that Google Maps doesn't quite have the blue dot in the right place.

There's quite a few reasons GPS might be inaccurate or slow. Here's a few choice excuses, we mean, explanations: delays as the signal passes through the atmosphere; reflections from tall buildings or large rocks; an inaccurate time on the receiver's clock; buildings or nature getting in the way and tight grouping of satellites.

So there you have it, we hope you have a new found respect for GPS and how hard the tech works to get its signals where they need to in order to precisely track your 5Ks round the park. GPS, we salute you.

2 Comments

23-Jul-2015 9:13 am

TafThorne says:

I think the process you describe is correct but you have named it incorrectly. Triangulation is the measurement of angles, trilateration is the measurement described which uses distances. Many people use the term triangulation in references to the GPS system as it is a more familiar word, if you are going to describe the process you should use the more correct term for it though.

You also mention that the satellites were put up there in the '70s but there are no active GPS satellites from that time left. That is the time most of the technical design was frozen and satellites were first launched. The current constellation consists of Block IIA and later launches, meaning the oldest satellites in use are from the '90s. There is a nice table summarising the currently active GPS Satellites on wikipedia.

In the excuses section you miss one of the biggest problems, obscuration. As this is a weak line of site signal that tall building or large rock might mean you cannot see a satellite at all. With a clear 360 degree view of the sky your receiver has the best chance of guessing were a satellite is and being able to see it. With tall buildings around you little of the sky is visible, you are in an urban canyon, so its guesses might be causing it to look at buildings instead of sky.